The condensation reaction of an olefin or a mixture of olefins over an acid catalyst to form higher molecular weight products is a widely used commercial process. This type of condensation reaction is referred to herein as an oligomerisation reaction, and the products are low molecular weight oligomers which are formed by the condensation of up to 12, typically 2, 3 or 4, but up to 5, 6, 7, or even 8 olefin molecules with each other. As used herein, the term ‘oligomerisation’ is used to refer to a process for the formation of oligomers and/or polymers. Low molecular weight olefins (such as propene, 2-methylpropene, 1-butene and 2-butenes, pentenes and hexenes) can be converted by oligomerisation to a product which is comprised of oligomers and which is of value as a high-octane gasoline blending stock and as a starting material for the production of chemical intermediates and end-products. Such chemical intermediates and end-products include alcohols, acids, detergents and esters such as plasticiser esters and synthetic lubricants. Industrial oligomerisation reactions are generally performed in a plurality of tubular or chamber reactors. Solid phosphoric acid, ion exchange resins, liquid phosphoric acid, sulphuric acid, molecular sieves, and zeolites, are known catalysts for oligomerisation.
Zeolites and other molecular sieves have become useful as catalysts for the oligomerisation of olefins. Zeolite oligomerisation processes have been disclosed in a number of documents too high to allow their reference in this document.
Industrial hydrocarbon conversion processes employing zeolite catalysts typically run for several weeks or months before a catalyst change is required or a decommissioning of the reactor is needed. There is a general desire to increase run length to increase catalyst use and to reduce the amount of down time. However it is necessary to balance increasing the run length with the production of the desired product. Various attempts have been made to accomplish this, such as by the development of new catalysts or the control of temperature and pressure in the reactors, as described in WO 2007/006398, and/or the space velocity, as described in WO 2008/074511.
The molecular sieves, typically in their acid form, have been replacing solid phosphoric acid (sPA) catalysts that have been the oligomerisation catalysts of choice for over half a century, since their introduction in the 1930's. Molecular sieves have particularly been successful in replacing solid phosphoric acid catalysts used for oligomerisation in tubular reactors. We have now found that the conventional procedure for starting up a tubular reactor, as it was used on solid phosphoric acid catalyst, leads to problems with zeolite oligomerisation catalysts.
Tubular oligomerisation reactors are designed like a shell and tube heat exchanger and are typically mounted with the tubes in a vertical position. The catalyst is usually located as a fixed bed of catalyst particles inside the tubes of the reactor, and the reactor is cooled with a heat transfer fluid on the shell side. Most conveniently the heat of the reaction is removed by vaporising or boiling up water on the shell side of the reactor. Suitable water for steam generation is a most readily available in a chemical plant, water has a very high heat of vaporisation compared to possible alternatives, and a boiling up regime provides a very high heat transfer rate from the tube wall. For that purpose, the shell side of the reactor is typically connected with a steam drum, from which water for the reactor is obtained as a liquid and into which the mixture of water and boiled up steam, which is generated in the oligomerisation reactor, is returned. The returning steam and water separate inside the steam drum, steam is removed from the steam drum, typically under pressure control, and make-up water is supplied, typically under level control, from a higher pressure source, to maintain an inventory of water in the steam drum. The steam is typically directed into a steam main or header, which operates at a typically constantpressure level and connects the appropriate steam generators with the appropriate steam consumers that are geographically sufficiently close. A chemical plant may comprise several steam mains or headers, which are typically maintained at different pressures. Steam at a higher pressure is typically of a higher value than steam at a lower pressure. Most of its heat content is in the latent heat, i.e. the heat of condensation. The condensation temperature of higher pressure steam is higher than this of lower pressure steam, enabling it to provide heat at a higher temperature level, which is more precious. Steam may be let down from a higher pressure main to a lower pressure level, simply through a control valve, or preferably through a turbine which can at the same time provide mechanical work.
During operation of the oligomerisation reactor, the catalyst typically looses activity over time, and it is customary to compensate for the activity loss by increasing the temperature in the reactor. With tubular reactors, this is conveniently accomplished by raising the pressure in the steam drum, such that the boil up temperature on the shell side of the reactor increases, which then increases the temperature in the catalyst bed located inside the tubes of the reactor. WO 2007/006398 describes how typically a temperature profile with a peak develops along the length of a reactor tube, and teaches to control the height of the peak temperature to no more than 50 degrees C. above the temperature of the temperature control fluid as this fluid exits the reactor, in order to achieve a high reactor runlength. WO 2008/074511 describes how the runlength of a zeolite oligomerisation reactor may be extended by running the reactor with a higher-than-average space velocity early in the run, and reducing the space velocity as the temperature increases further through the run. WO 2008/074511 further describes how the reactor runlength may be further extended by reducing the vulnerability of the reactor operations to unintentional variations in the enthalpy balance as it approaches its end-of-run. These publications are concerned with optimising normal operations for a tubular reactor throughout its run, and extending the length of the run. They are silent about the start up phase of the reactor, i.e. the period of reactor operation before stable early-run conditions are obtained. We have found that the start up method may be improved, such that overall production of desired products and total runlength of a reactor may be increased.
We have found that molecular sieve catalysts, either when fresh or freshly regenerated, are characterised by a surprisingly high activity. This activity is much higher than the activity of fresh sPA catalysts, because sPA catalysts can only reach their maximum activity once the ideal hydration conditions are achieved for the temperature the catalyst operates at, and because there is a high inertia in the hydration mechanisms and their controls. EP 1118651 discloses a process using solid phosphoric acid catalyst using a start-up fluid that is anhydrous, with the purpose to bring the catalyst to its desired hydration conditions sooner.
With fresh zeolite or molecular sieve catalysts, all active sites are inherently active from the start of the contacting with the olefin, and a maximum number of active sites are available when the reactor is started up.
We have found that the conventional start up method for a tubular reactor is not appropriate for a reactor containing fresh zeolite or molecular sieve catalysts. Following the conventional start up method, the reaction is allowed to progress too fast and too far, and produces much heavier oligomers than those that are desired, usually referred to as “heavies”, and partially to the formation of even heavier asphalt-like heavy byproducts, also known as “coke”. This uncontrolled overshoot leads to a fast coking up of the most active sites on the catalyst, which thus become unavailable for further participation in the oligomerisation reaction even before stable early run conditions have been established. The longer the overshoot lasts, the more damage is caused to the molecular sieve catalyst, and the more undesired heavier oligomers are produced.
There therefore remains a need for an improved start up procedure, whereby stable early run conditions may be reached with a reduced loss of the more active sites on the catalyst, and while more desired oligomer products may be produced instead of heavies. The present invention aims to obviate or at least mitigate the above described problem and/or to provide improvements generally.